The present invention relates to a planar transformer providing low output voltage, particularly a planar transformer for use in DC-DC power converters.
As microprocessors and other integrated semiconductor devices become denser, it is desirable to lower their supply voltage requirements. Accordingly, supply voltages for digital electronics have been reduced from 5 volts to 3.3 volts, then to 2.5 volts, and are now being reduced to 1.6 volts. The trend is expected to continue.
A power converter for supplying the integrated circuits typically employs a transformer to accept an input at a relatively high voltage and produce a lower output voltage. As is well known, standard transformers typically employ a ferrite core around which wires forming primary (input) and secondary (output) circuits are wound. These standard transformers are notoriously expensive and bulky however, and alternative transformer embodiments have been used in computer power supplies to lower cost and decrease size. One such embodiment is the xe2x80x9cplanarxe2x80x9d transformer, wherein the wires are replaced by traces in one or more layers of a circuit board.
The prior art transformer has a limitation in the output voltage that it is capable of producing. Particularly, magnetic technology has typically been limited to the output voltage that is produced by one turn of the secondary. To address this limitation, fractional turns have been employed. Referring to FIG. 1 as an example, a core 2 is shown having a center leg 3 around which a primary winding 4 is looped. The core 2 has two secondary xe2x80x9clegsxe2x80x9d 6a and 6b, and a secondary winding 8 is looped around one of the legs 6a one half-turn. A problem with this transformer is that magnetic flux circulating from the center leg through the other leg 6b leads to an undesirable leakage inductance.
To address this problem, referring to FIG. 2, a half-turn of the secondary 8 is looped around the leg 6a and a half-turn is looped around the leg 6b. The two loops contribute to the total output voltage in parallel, and all of the flux in the core links the secondary. However, a problem remains in that the two legs 6a and 6b are not identical, so that the magnetic flux through the respective half-turns is not identical. In response, an additional circulating current flows in the secondary in order to balance the magnetic flux, leading to additional ohmic power loss.
Another problem with the prior art as shown in FIG. 2 is that the secondary 8 winding is relatively long compared to the secondary winding shown in FIG. 1. This also increases ohmic loss in the transformer, and in addition increases stray inductance.
Accordingly, there is a need for a transformer providing low output voltage that provides for converting substantially all the magnetic flux circulating in the core of a transformer into an output current, particularly by decreasing ohmic loss and stray inductance.
The transformer providing low output voltage of the present invention solves the aforementioned problems and meets the aforementioned needs by providing a magnetic core having at least two apertures defining a center portion between the apertures and two leg portions. The core has primary and secondary windings. The primary winding receives a first voltage or current and induces a second voltage or current in the secondary winding. The input power is typically though not necessarily provided at a higher voltage than the output power, the latter which is preferably less than or substantially equal to 3.3 volts.
The primary winding has a first portion looped around one of the leg portions so that a current passed through the first winding will produce a magnetic flux in that leg portion that circulates in either the right hand or left hand sense. A second portion of the primary winding is looped around the other leg portion in the opposite sense. This provides for a magnetic flux circulating through the two outer leg portions in the same sense, and provides that the magnetic flux circulating through the center portion is zero. The secondary winding is preferably provided as a fractional loop around one of the outer leg portions.
Therefore, it is a principal object of the present invention to provide a novel and improved transformer providing low output voltage.
It is another object of the present invention to provide a transformer providing low output voltage that provides for converting substantially all the magnetic flux circulating in the core of a transformer into an output current.
It is yet another object of the present invention to provide a transformer providing low output voltage that provides for high efficiency.
It is still another object of the invention to provide a transformer providing low output voltage that provides for minimal leakage inductance.
It is a further object of the present invention to provide a transformer providing low output voltage that provides for minimal ohmic loss.
It is still a further object of the present invention to provide such a transformer at lower cost.
The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings.